1. Technical Field
The present invention relates to a method for producing a polarizing element.
2. Related Art
As one type of polarizing element, a polarizing glass is known. A polarizing glass can be composed of only an inorganic substance, and therefore, as compared with a polarizing plate containing an organic substance, the deterioration thereof due to light is significantly less. Therefore, a polarizing glass has drawn attention as an effective optical device in a liquid crystal projector whose brightness has been enhanced recently.
As a general polarizing glass, those described in JP-A-56-169140 are known, and a method for producing such a polarizing glass is as follows.
(1) A glass product having a desired shape is produced from a composition containing silver and at least one halide selected from the group consisting of chlorides, bromides, and iodides.
(2) The produced glass product is heated to a temperature which is higher than the strain point but not higher than the softening point of the glass by about 50° C. for a period of time sufficient to produce crystals of AgCl, AgBr, or AgI in the glass product, whereby a crystal-containing product is produced.
(3) The resulting crystal-containing product is elongated under stress at a temperature which is higher than the annealing point but lower than a temperature at which the glass has a viscosity of about 108 poises so that the crystals are elongated to have an aspect ratio of at least 5:1.
(4) The elongated product is exposed to a reducing atmosphere at a temperature which is higher than about 250° C. but not higher than the annealing point of the glass by about 25° C. for a period of time sufficient to develop a chemically reduced surface layer on the product. By this process, at least a portion of the elongated silver halide particles are reduced to elemental silver.
Meanwhile, there is also known a method in which after silver or copper is introduced into a glass surface layer by an ion exchange method, a silver or copper halide phase is deposited, and the deposited halide phase is elongated, thereby forming a layer having a polarization separation function on the surface layer of a glass product (see Japanese Patent No. 4394355).
According to the production method described in JP-A-56-169140, the halide deposits uniformly in the glass product, however, in the reducing step, the halide only in the surface layer of the glass product can be reduced, and therefore, the halide remains in a central portion in the thickness direction of the glass product. Due to this, the transmittance of a polarizing element is decreased and when the glass product is applied to a liquid crystal display apparatus or the like, there is a possibility that a sufficient brightness cannot be obtained.
On the other hand, according to the method described in Japanese Patent No. 4394355, silver or copper is introduced only into a surface layer portion of the glass product, and therefore, the above problem caused by the halide remaining without being reduced can be prevented. However, it is necessary to immerse the glass product in a molten salt at a high temperature (350 to 750° C.) for as long as about 8 hours, and therefore, an environmental load is large. In other words, the consumption energy for the production is extremely large and the productivity is low.